Phase and frequency modulation



March 2, 1948.

Filed Nov. 17, 1942 3 Sheets-Sheet 1 A c o F G L|M|TER LIMITER SOURCE OFGOMBINER couamzn g To ANY FIXEDPHASE QO'PHASE OF CARRIER 90' PHhSE OFCARRIER AMTONAL AND SHIFTER flSIDE BAND SHIFTER a BAND Mg??? FREQUENCYVECTOR a vsc on a TRANSWTTEF';

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PHASE AND FREQUENCY MODULATION Filed Nov. 17, 1942 3 Sheets-Sheet 3 FIG.5

IN VEN TOR E EDWIN KQS'TODOLA Y Wad- Q.

ATTORNEY Patented Mar. '2, 1948 PHASE AND FREQUENCY MODULATION Edwin E.Stodola, Neptune, N. .1. Application November 17, 1942, Serial No.465,920

6 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) This invention relates to modulation systems, particularlythose of the phase and frequency modulation type.

Most phase modulation systems permit change of phase which is linearwith respect to modulation voltage over a relatively small range ofphase displacement. To obtain larger phase shifts, it has been thepractice to pass the phase shifted wave through a series of multipliersand heterodyne stages to get a desired phase shift at a desired value ofmean carrier frequency. This results in a large amount of complicationand a loss in frequency stability of the final carrier wave.

It is an object of this invention to devise a phase or frequencymodulation network in which linear phase displacement over larger rangesis made possible, thus permitting a reduction in the number of frequencymultipliers and the elimination of some or all of the heterodyningstages.

More specifically, it is an object of this invention to provide acascaded modulation system in which the first stage provides a portionof the phase shift and the resultant phase shifted wave is furthershifted in phase by the succeeding stages.

It is a further object of the invention to provide a novel cascadedphase or frequency modulation system wherein undesirable amplitudemodulation is eliminated or neutralized.

Further objects will be apparent from the specification and drawings,wherein Figure 1 is a simplified diagram of one form of I modulationsystem.

Referring to Figure 1, the output of a relatively fixed frequencycarrier voltage source A is applied to a balanced amplitude modulator Bupon which the modulating wave is also im= pressed. The modulator B isof the type in which part, and preferably all, of the carrier issuppressed and the output of the modulator contains modulation sidebands the amplitude of which vary in proportion to the modulatingvoltage amplitude. Modulator B may be any conventional type of carriersuppressing modulator or it may be of the type described and claimed inmy copending application entitled Modulation networks, Serial No.465,922, filed on even date herewith.

The output of carrier source A is also passed thru a fixed phase shifterC and then combined with the output of modulator B in combining networkD. The resultant output will be a phase modulated wave the meanfrequency of which is the same as that of source A. Network D may alsocontain conventional buffer amplifiers and amplitude limiters to removeany undesirabie amplitude modulation that may be present.

As thus far described, the system is conventional. With such a system itis, however, possible to obtain phase shifts which are substantiallylinear with respect to modulation voltage through a range of only aboutthirty degrees. To provide large phase shifts it has been-the custom tomultiply the frequency of the phase shifted wave several thousand times.Since this process also multiplies the mean frequency of the carrier ithas been the practice to interpose in the muitiplying stages one or morehetercdyning stages wherein the output of a fixed oscillator isheterodyned with the multiplied frequency and the difference frequencyis again multiplied and/or amplified and finally radiated.

However, this process involves use of two or more oscillators and thefrequency stability of the final carrier is the algebraic sum of thestabilities of all the oscillators. Consequently, the number ofheterodyning stages used and therefore the maximum amount of phase shiftthat can be obtained with this method is limited, if a very stablecarrier frequency is required, unless resort is had to complicated andcumbersome techniques.

In the present invention, the substantially linear phase shift can bealmost doubled by sub- J'ecting the phase modulated ouput of network I)to a further modulation in networks E, F, and G which function in thesame manner as networks B, C, and D respectively. The same modulatingwave controls both modulators B and E. Any number of phase modulatorscan be connected cascade as above described. Thus large linear phaseshifts can be'obtained from one carrier source, permitting the reductionin the number of frequency multiplication stages and reduction orelimination of the number of heterodyning stages.

Figure 2 illustrates another embodiment of my invention in which use ismade of a type of phase modulator such as is described and claimed in mycopending application, entitled Modulation system, Serial No. 652,210,filed March 5, 1945, which is in turn a division of my applicationentitled Electrical networks, Serial No. 465,921, filed November 7,1942, which has matured into Patent No. 2,397,992, issued April 9, 1946.For an eX plantion of this modulator, reference is made to Figure 3.

If a fixed voltage V is impressed upon a reactor 29 in series with aresistor 3, the output voltage V will be phase shifted with respect to Vby an angle whose tangent is equal to the ratio of the reactance dividedby the resistance. If such reactance is capacitative the output voltagewill lead the input voltage. If the reactance is inductive, it will lagthe input voltage.

If resistor 3 is the anode-cathode space of a triode, or other form ofelectron tube, to the grid of which is applied a modulating voltage, andinput voltage V is derived. from a carrier source, then the outputvoltage V0 will be phase modulated. However, the phase of the outputvoltage will not, in general, be a linear function of the modulatingvoltage, but the function can be made substantially linear over theoperating range by choosing the proper value of series reactance for agiven tube resistance.

Figure 4 shows a family of curves 3iia3td, representing the change inphase angle between the input voltage V and the output voltage V0 withrespect to changes in grid bias of a type 605 tube having differentvalues of reactance in series with the anode-cathode path of the tube.In the curves shown, the tube is operated with a plate polarizingpotential of 250 volts D. C.

Curve 30b shows that with a reactance of 15,009 ohms in series with thetube a linear relationship exists between grid voltage and phase changefor a range of 15 to 55.

Referring to Figure 2, the output of a relatively fixed frequencyoscillator A is applied to a phase modulating network J, similar to thatshown in Figure 3, in which the reactance 29 is a condenser 29a and thevariable resistor 3 is the anode-cathode path of a triode 3a connectedin series with condenser 29a. A source of D. C. plate potential I5 isconnected to the plate of the triode through an R. F. choke [6. Toneutralize or minimize the effect of the anode-cathode capacity of thetriode, a condenser l8 and inductance I1 is connected across the anodeand cathode path to form with said path a parallel resonant circuitwhich is resonant at, and presents a very high impedance to, thefrequency of the current impressed thereon from oscillator A. A blockingcondenser l5, of negligible impedance to the impressed frequency isinterposed in the anode circuit to prevent shorting of the plate batterythrough coil l'l.

The output of modulator J is impressed upon a second modulator Mdirectly or through one or more buffer amplifiers K. Condenser I9 is ablocking condenser of negligible impedance to the impressed frequency.

Modulator M is similar to modulator J, except that reactance 29b isinductive. The phase shifting characteristics of the two modulators aretherefore opposite in sense.

To make the total phase modulation cumulative, the grids of the twomodulators are driven in opposite phase by a differential modulatincircuit including the push-pull secondary oi transformer 2t, on theprimary of which is im pressed the modulating voltage; A steady negativebias is impressed upon the grids of both modulator grids by battery 2iconnected between ground and the center-tap of the transformersecondary.

The output of modulator M can also be impressed upon furthercascade-connected phase modulators, such as above-described, and thenfrequency multiplied and radiated. Or it may first be frequencymultiplied and then passed through more phase modulator stages operatingat the multiplied frequency. Amplitude limiting can also be introducedat any stage in the cascade if undesirable amplitude modulation ispresent.

The circuit in Figure 2 permits of another ad vantage. Most phase orfrequency modulators also amplitude modulate the impressed wave. Thisundesirable by-product is usually eliminated by one or more amplitudelimiting stages. Reference to curve see in Figure 4 will indicate theamount of amplitude variation when reactance 29 has a value of 15,000ohms. By modulating each pair of modulators in opposite phase, as isdone in Figure 2, the amplitude modulation is largely cancelled. Thismethod of reducing the amplitude modulation can also be applied to asingle stage phase modulator. This can be done by impressing themodulation voltage in reversed phase upon an amplitude modulator uponwhich the phase modulated wave is impressed, e. g. upon the control gridof the tube in circuit K of Figure 2.

The curves in Figure 5 show the characteristics of the systemabove-described compared with those of a single stage modulator. Curves5a and 50 show the range of amplitude and linear phase variation,respectively, of a single unit modulator. Curves 5b and 512,respectively, show the smaller amount of undesirable amplitudemodulation and the larger amount of linear phase modulation of the novelsystem herein described.

It should be noted, in general, that phase modulators are not linearover their whole operating range. Where several like modulators arecascaded, their non-linearities and amplitude modulators are cumulative.It is therefore desirable, where a large number of modulators arecascaded, to use modulators having undesirable characteristics whichtend to neutralize each other. In Figure 2 this has been applied particularly with respect to amplitude modulation.

Although reference has been made to phase modulation, it is appropriateto refer to the fact that pure frequency modulation is equivalent tophase modulation in which phase shift for a given modulating voltage isinversely proportional to the modulating frequency. At any singlemodulating frequency there is no distinction between the two.Consequently, where reference has been made to phase moduation, itshould be understood that by appropriate control of the modulatingvoltage the same methods and devices can be used to produce frequencymodulation. In the claims, the term wave-length modulation Will be usedas generic to both types of modulation.

It should be understood that the specific circuits above described aremerely illustrative of the principles of the invention. Manymodifications may be made without departing from amazes the spirit ofthe invention as defined in the appended claims.

Iclaim:

1. A wave-length modulation system for a source of oscillations ofrelatively fixed frequency comprising a pair of phase modulators, one ofsaid modulators comprising a capacitor and resistor connected in seriesto said source, a coupling circuit connected across said resistor and tothe other modulator, said other modulator comprising an inductor andresistor connected in series to said coupling circuit, and means forvarying the resistances of said resistors in opposite senses.

2. A system as set forth in claim 1, wherein the reactances of saidcapacitor and. inductor, at the frequency of said source, are equal andthe resistances of said resistors are identical,

3. A system as set forth in claim 1, wherein each resistor isconstituted by the anode-cathode path of an electron tube.

4. A system as set forth in claim 1, wherein each resistor isconstituted by the anode-cathode path of an electron tube and means forreducing the effect of the anode-cathode capacity.

5. A wave-length modulation system for a source of oscillationscomprising a first pair of series-connected impedances across saidsource, a second pair of series-connected impedances coupled across oneof the impedances of said first. 30

pair, an output circuit coupled across one of the impedances of saidsecond pair, one impedance in each pair being resistive, the remainingimpedance in one pair being inductive, the remaining impedance in theother pair being capacita- Ltive, and means to differentially vary saidresistive impedances.

6. A system as set forth in claim 5, wherein said inductive andcapacitative impedances are of equal reactance at the frequency of saidsource,

REFERENCES CITED The following references are of record in the .file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,045,107 Shore June 23, 19362,075,071 Usselman Mar. 30, 1947 2,104,318 Crosby Jan. 4, 1938 2,143,386Roberts Jan. 10, 1939 2,160,528 Usselman May 30, 1939 2,173,145 WirklerSept. 19, 1939 2,231,079 Longo Feb. 11, 1941 ---2,254,734 Falloon et a1,Sept. 2, 1941 2,281,935 Hansen May 5, 1942 2,294,372 Barton Sept. 1,1942 2,321,269 Artzt June 8, 1943 Certificate of Correction Patent No.2,436,834. March 2, 1948. EDWIN K. STODOLA It is hereby certified thaterror appears in the printed specification of the above numbered patentrequiring correction as follows: Column 4, lines 49 and 50, for the wordmodulators read modulations; and that the said Letters Patent should beread with this correction therein that the same may conform to therecord of the case in the Patent Office.

Signed and sealed this 1st day of June, A. D. 1948.

THOMAS F. MURPHY,

Assistant Oommtisszoner of Patents.

